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Author: Grady, Brian P. × Author: Pötschke, Petra ×

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    Electrical, mechanical, and glass transition behavior of polycarbonate-based nanocomposites with different multi-walled carbon nanotubes
    (Oxford : Elsevier Science, 2011) Castillo, Frank Yepez; Socher, Robert; Krause, Beate; Headrick, Robert; Grady, Brian P.; Prada-Silvy, Ricardo; Pötschke, Petra
    Five commercially available multi-walled carbon nanotubes (MWNTs), with different characteristics, were melt mixed with polycarbonate (PC) in a twin-screw micro compounder to obtain nanocomposites containing 0.25-3.0 wt.% MWNT. The electrical properties of the composites were assessed using bulk electrical conductivity measurements, the mechanical properties of the composites were evaluated using tensile tests and dynamic mechanical analysis (DMA), and the thermal properties of the composites were investigated using differential scanning calorimetry (DSC). Electrical percolation thresholds (pcs) were observed between 0.28 wt.% and 0.60 wt.%, which are comparable with other well-dispersed melt mixed materials. Based on measurements of diameter and length distributions of unprocessed tubes it was found that nanotubes with high aspect ratios exhibited lower pcs, although one sample did show higher pc than expected (based on aspect ratio) which was attributed to poorer dispersion achieved during mixing. The stress-strain behavior of the composites is only slightly altered with CNT addition; however, the strain at break is decreased even at low loadings. DMA tests suggest the formation of a combined polymer-CNT continuous network evidenced by measurable storage moduli at temperatures above the glass transition temperature (T g), consistent with a mild reinforcement effect. The composites showed lower glass transition temperatures than that of pure PC. Lowering of the height of the tanδ peak from DMA and reductions in the heat capacity change at the glass transition from DSC indicate that MWNTs reduced the amount of polymer material that participates in the glass transition of the composites, consistent with immobilization of polymer at the nanotube interface. © 2011 Elsevier Ltd. All rights reserved.
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    Aspect ratio effects of multi-walled carbon nanotubes on electrical, mechanical, and thermal properties of polycarbonate/MWCNT composites
    (Hoboken, NJ [u.a.] : Wiley, 2014) Guo, Jiaxi; Liu, Yanjun; Prada-Silvy, Ricardo; Tan, Yongqiang; Azad, Samina; Krause, Beate; Pötschke, Petra; Grady, Brian P.
    Two multi-walled carbon nanotubes (MWCNTs) having relatively high aspect ratios of 313 and 474 with approximately the same diameter were melt mixed with polycarbonate (PC) in a twin-screw conical micro compounder. The effects of aspect ratio on the electrical, mechanical, and thermal properties of the PC/MWCNT composites were investigated. Electrical conductivities and storage moduli of the filled samples are found to be independent of the starting aspect ratio for these high aspect ratio tubes; although the conductivities and storage moduli are still significantly higher than values of composites made with nanotubes having more commercially common aspect ratios of ∼100. Transmission electron microscopy results suggest that melt-mixing reduces these longer nanotubes to the same length, but still approximately two times longer than the length of commercially common aspect ratio tubes after melt-mixing. Molecular weight measurements show that during melt-mixing the longer nanotubes significantly degrade the molecular weight of the polymer as compared to very similar nanotubes with aspect ratio ∼100. Because of the molecular weight reduction glass transition temperatures predictably show a large decrease with increasing nanotube concentration. © 2013 Wiley Periodicals, Inc.